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SUMMARY:Trees for subtoposes of the effective topos - Sori Lee\, DPMMS
DTSTART:20111025T131500Z
DTEND:20111025T141500Z
UID:TALK33951@talks.cam.ac.uk
CONTACT:Julia Goedecke
DESCRIPTION:For making concrete calculations about subtoposes of the effec
 tive topos\, it is crucial to have a tangible representation of the corres
 ponding local operators. We introduce tree-like structures called `sights'
 \, in order to further understand a representation due to A.M. Pitts (whic
 h arises from the topos-theoretic construction of the least local operator
  making a given subobject of a topos dense) and to define another useful r
 epresentation. We apply these methods to establish some new examples of su
 btoposes\, and to compare them to known ones. We also remark that a realiz
 ability-like semantics can be formulated for the arithmetic of subtoposes 
 in terms of sights.
LOCATION:MR5\, Centre for Mathematical Sciences
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